Low-melting metals which are used as solder or solder constituents are generally very prone to oxidation. Therefore, when producing joining connections by means of a solder which contains a low-melting metal, the formation of oxide at the surface of the solder layer often impedes complete wetting of one of the joining partners. Open, so-called cold soldering locations or soldered connections which are only connected at a few points may ensue. The formation of oxide can be reduced by soldering under a shielding gas or a reducing atmosphere. However, this entails increased production costs. Furthermore, the oxidation can also be reduced by using flux. However, this is often not practicable or desirable in particular in the case of thin soldered connections or large-area joins.
Document DE 103 50 707 describes an electrical contact for an optoelectronic semiconductor chip, in which a metal layer on the semiconductor surface produces an electrical metal/semiconductor contact. To avoid diffusion of constituents of the solder layer into the metal layer applied to the semiconductor surface, which preferably acts as a mirror for the radiation emitted from the semiconductor chip, a barrier layer, for example a barrier layer of TiWN, is included between the solder layer and the metal layer.
The problem often also exists that solder layers have poor adhesion to a barrier layer of this type. Furthermore, solder layers often also have poor adhesion to metallic carriers. The reason for this is the poor wettability of the barrier layers and/or the surface of the carrier. For this reason, a layer with good wetting properties, for example a wetting layer of platinum is introduced between the barrier layer and the solder layer or between the carrier surface and the solder. In this case, however, there is a risk of the material of the wetting layer, in particular platinum, reacting at least slightly with the solder. In the case of thin solder layers, this can disadvantageously alter the stoichiometry of the solder.
Furthermore, it is known from document DE 103 50 707 to protect the solder layer from oxidation by applying a thin layer of gold.
With some solder metals, however, there is a risk of solder constituents, in particular low-melting metals, diffusing through an oxidation prevention layer of this type even at low temperatures, for example even at room temperature, thereby reducing the protective effect. If the thickness of the protective layer were to be significantly increased, in order to prevent diffusion of a solder constituent toward the surface of the oxidation prevention layer, the thickness of the solder layer would also have to be increased accordingly if the composition of the solder is not to be significantly altered by the protective layer following the melting process. However, in particular for the soldering of semiconductor chips, for example for soldering LED chips in an LED housing, low solder thicknesses are generally required in order to minimize the overall height of the component and to rule out the risk of short circuits at the edge of the chip.